In the wet-pressing mode of papermaking machines, a substantial part of the residual liquid still in the paper web is pressed out between compression rollers forming a compression slit. The paper web is made to pass through the compression slit by means of a revolving wet felt, the liquid being pressed out of the paper web in the compression slit into the wet felt and then being drained.
Recently wet presses with extended compression slits, the so-called "shoe presses" have been developed, where the paper web is exposed over a longer path and hence over a correspondingly longer time to high compressions so that it shall exit the wet press with less moisture. Belts are employed to guide the paper web and at least one wet felt through such an extended slit, said belts comprising a flexible belt layer impermeable to liquids and smooth on its back side. By means of this smooth back side said belts pass over a hydraulically loaded pressure pad in the compression slit which will press the belt toward an opposite compression roller. The paper web to be dehydrated is made to pass between this compression roller and the belt and is accompanied at least on one side by a co-moving wet felt draining the pressed-out water.
Such belts are subjected to high stresses in the longitudinal and transverse directions when in the compression slit and they also undergo more than trivial abrasion on both sides. To solve the former problem, it has been suggested to fully integrate a fabric acting as a support track into the belt layer (German Offenlegungsschriften 32 31 039 and 33 18 984; U.S. Pat. No. 4,559,258). However these designs have failed the test of practice.
In order to improve the water drainage from the front side of the belts, that is from the side facing the paper web and resting against a co-moving wet felt, the front side is textured. Belts have been developed for that purpose in which support tracks were integrated only in part on the front side, so that they partly project above the belt layer. As a result cavities and ducts were created for water drainage. Multi-ply fabrics have been suggested as support tracks (German patent 32 35 468; European patent 0 098 502; European Offenlegungsschrift 0 138 797; German Gebrauchsmuster 83 19 684.6; European Offenlegungsschrift 0 185 108). In lieu of fabrics, belts of wire-links also have been proposed (European patent 0 098 502) or knits (European Offenlegungsschrift 0 290 653). As regards the embodiment of the support track as a wire-link belt, it was suggested additionally to make the mutually coupled wire spirals longitudinal and to additionally insert into them multifilaments and/or monofilaments to absorb the longitudinal forces (European patent 0 098 502). The purpose of their introduction into the belt layer is to improve belt dimensional stability and also the adhesion of the wire spirals to the belt layer.
Where the belts support tracks were fabrics partly integrated into the belt layer, it has been additionally suggested to design the outwardly projecting part of the fabric as the wear layer and the part of the fabric integrated into the belt layer as the traction-absorbing base-layer (German Gebrauchsmuster 83 19 684.6). It is moreover in the state of the art of such belts to select part of the longitudinal threads to be made of a material exceedingly dimensionally stable longitudinally and to make the other part of the longitudinal and also transverse threads from a highly wear-resistant material (European patent 0 185 108). The purpose is to achieve better wear resistance, pressure distribution, longitudinal dimensional stability and rolling resistance.
Just as it was found that complete integration of a fabric (German Offenlegungsschrift 3 231 039) into the belt layer was unsatisfactory in practice, so it was found it is inappropriate to make the only partly integrated support track enter too deeply the belt layer and, in the extreme case, to have it stretch over the entire cross-section of the belt layer (German patent 32 35 468). Because of the high compression and compression stresses with different forces exerted at the belt's front and back sides, especially in the compression slit, the bond between the support track and the belt layer is not permanently secured. Furthermore, the belt layer requires an extraordinary thickness so that in spite of its deep integration into the cross-section of the belt layer it shall still project from it in order that a cavity volume required for water drainage be formed. However so thick a support track entails the belt becoming stiff and therefore shall only poorly adapt to the shape of the of the pressure pad. Again, the manufacture of such a support track is complex and hence costly.
In view of the above, the support track preferably is integrated only slightly, that is no more than into half the thickness of the belt layer (European patent 0 098 502). This entails however a tradeoff in that the support more easily is torn out of the belt layer. Even the additional longitudinal threads cannot prevent this consequence because being integrated only into the edge zone of the belt layer. Nor do these longitudinal threads prevent the formation of a compression bead in front of the pressure pad as seen in the direction of advance, and moreover they absorb longitudinal forces only in a limited way because anyway being in a zone which is tension-relieved by the support track. Accordingly a comparatively hard material is required for the belt layer so that the above compression bead shall not occur. As a consequence, the belt is comparatively stiff and only incompletely adapts to the shape of the pressure pad.
The object of the invention is to create a belt of the above kind which on one hand can withstand high traction and compression stresses but on the other hand evinces high flexibility and therefore adaptability to particulars, especially in the compression slit.